The trait `Clone` is not implemented for `&mut T`

help
#1

Hi, Getting the error: the trait Clone is not implemented for &mut T. Any help or pointer please?

pub trait Indicator
{
    type Item;

    fn get_input_series(&self) -> CandleSeries;

    fn get_input_series_len(&self) -> usize;

    fn get_value(&mut self, index: usize) -> Option<Self::Item>;

    fn get_last_value(&mut self) -> Option<Self::Item> {
        let len: usize = self.get_input_series_len();
        self.get_value(len - 1)
    }

    fn get_all_values(&mut self) -> Vec<Option<Self::Item>>;

    fn calculate(&mut self, index: usize) -> Option<Self::Item>;

    fn calculate_all(&mut self) -> Vec<Option<Self::Item>>;
}

#[derive(PartialOrd, PartialEq, Clone, Debug)]
pub struct EMAIndicator<'ema, T>
    where T: Indicator + Clone {
    indicator: &'ema mut T,
    period: usize,
    multiplier: f64,
    calc_indicator_values: Vec<Option<f64>>,
}

error[E0277]: the trait bound `&mut T: Clone` is not satisfied
   --> src/traits.rs:281:5
    |
281 |     indicator: &'ema mut T,
    |     ^^^^^^^^^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `&mut T`
    |
    = help: the following implementations were found:
              <&T as Clone>
              <&mut T as Clone>
    = note: `Clone` is implemented for `&T`, but not for `&mut T`
    = note: required by `clone`
    = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)
#2

This can't possibly work. Cloning a mutable reference would create two mutable references to the same location. This is not allowed. There must either be a single mutable reference or any number of immutable references.

2 Likes
#3

Solution? How to approach this in Rust?

#4

What is it exactly that you want? Why does EMAIndicator need to be cloned and why is indicator a mutable reference instead of immutable reference?

#5

I follow what you are saying, and the reason I want to Clone it is because I have PriceIndicator which is then used to create multiple EMAIndicator inside MACDIndicator. So basically, how to address this problem for the new of MACDIndicator i.e. how do I use the Indicator field and the other field such as short_term_period and long-term_period. Also Mutable Reference because they contain vector which holds the cached results to avoid re-calcs.

#[derive(PartialOrd, PartialEq, Clone, Debug)]
pub struct EMAIndicator<'ema, T>
    where T: Indicator + Clone {
    indicator: &'ema mut T,
    period: usize,
    multiplier: f64,
    calc_indicator_values: Vec<Option<f64>>,
}

#[derive(Debug)]
pub struct MACDIndicator<'macd, T>
    where T: Indicator + Clone {
    indicator: &'macd mut T,
    short_term_period: usize,
    long_term_period: usize,
    signal_line_period: usize,
    calc_indicator_values: Vec<Option<f64>>,
    calc_macd_values: Vec<Option<f64>>,
    calc_signal_values: Vec<Option<f64>>,
    calc_histogram_values: Vec<Option<f64>>,
    short_term_ema_indicator: EMAIndicator<'macd, T>,
    long_term_ema_indicator: EMAIndicator<'macd, T>,
}

impl<'macd, T> MACDIndicator<'macd, T>
    where T: Indicator + Clone {
    pub fn new(indicator: T) -> Self {
        Self {
            indicator,
            short_term_period: 12,
            long_term_period: 26,
            signal_line_period: 9,
            calc_indicator_values: Vec::new(),
            calc_macd_values: Vec::new(),
            calc_signal_values: Vec::new(),
            calc_histogram_values: Vec::new(),
            short_term_ema_indicator: EMAIndicator::new(indicator.clone(), 12),
            long_term_ema_indicator: EMAIndicator::new(indicator.clone(), 26),
        }
    }
#6

You could try to use &'ema RefCell<T>.

#7

Was trying to add RefCell. Is that the only possibility? Also from Design perspective it should be simpler than this and I may be doing something incorrect.

#8

That is about the only possibility. There are also types like Cell and once_cell::unsync::OnceCell that are more specialized, but also more limited. You will have to use interior mutability in some way to mutate through an immutable reference.

#9

oh ok, thanks. But this problem might crop in several places. If you happen to think of a better solution to this please let me know. Again thanks a lot for your time and help.

Warm Regards,

#10

And the reason I was trying to avoid RefCell is that it will be slower isn't it ?

#11

If you need to mutate some value from multiple places, there always be some kind of performance hit, to ensure the absence of data races,

1 Like
#12

yes kind of makes sense and I think I need to rethink my design a bit more. Should not have had to do this as I never change the values for the PriceIndicators. It's the wrappers with the calculated values that need to be mutable. Also, I am yet not sure, inside the new method, is there a possibility to refer to another field? Still need to read the Rust Book :slight_smile:

#13

Eek, that's not a helpful help. There's definitely not a Clone for &mut T impl. I've filed a bug about it:

9 Likes
#14

Thank you so much @scottmcm. Finding the Rust community here to be one of the most amazing!